This invention pertains to the manufacture of carbonaceous and graphitic articles such as the anodes, cathodes, and electrodes used in the aluminum and steel industries. Tars and pitches have been used as binders for carbonaceous aggregates for some time. This type of mix has found wide usage in making electrodes for the aluminum industry and in making refractories for use in basic oxygen steelmaking furnaces, electric arc steelmaking furnaces, and iron and steel ladles. The tars and pitches serve not only to bond the aggregate together during the manufacture process, but when the shaped mix is heated to a pyrolysis or carbonizing temperature, the pitch decomposes and carbon is deposited within the pore structure and around the aggregate. This carbon residue, resulting from pyrolysis of the pitch or tar at high temperature, forms a strong carbon bond between the aggregates.
The techniques used in the manufacture of tar or pitch bonded articles have been well established. For example, a size graded batch of aggregate is intimately mixed with pitch and other carbonaceous materials at 250.degree. to 300.degree. F. The resulting mixture is formed into shapes on a press. As the shapes cool, the pitch solidifies and forms a strong bond. Since tars and pitches are not thermosetting materials, they will again melt and soften and cause the shapes to become very weak in the 200.degree. to 600.degree. F. temperature range. If a shaped article of this type were used in a high temperature environment requiring mechanical strength, the shapes would be subject to fragmental breaking, spalling and undesirable subsidence.
In order to eliminate the problems associated with the softening of pitch bonded shapes in the temperature range of 200.degree. to 600.degree. F., resin binders have been sought as substitutes. Resins have been sought because they not only function as a pitch does in a mix in reference to binding the aggregate during molding, and yield a substantial amount of carbon upon pyrolysis, but also cure to a thermoset material. The ability of a resin to cure to a thermoset material eliminates the problem of the binder softening in the 200.degree. to 600.degree. F. temperature range.
While the use of resins solves the strength problem at low temperature, shapes bonded with resins do not exhibit a strengthening effect above 600.degree. F. as pitch bonded shapes do.
While the use of coal tar pitch as the binder in the manufacture of carbon/graphite anodes, cathodes, and electrodes for the metal industry has provided products with excellent physical and mechanical properties and continues to be used as a economically cheap bonding agent in the anode, cathode, and electrode industry, coal tar pitch binders involve serveral problems. One area of considerable concern is the environmental and personal hygiene problems associated with use of coal tar pitch. Coal tar pitch has come under considerable scrutiny due to its suspected carcinogenic properties. This has necessitated extensive environmental controls in the work area and extensive safety precautions to insure minimal worker exposure to coal tar pitch.
When coal tar pitch is used as a bonding agent, retention of shape and prevention of sagging of the molded articles is a problem, because of the inherent thermoplastic nature of the coal tar pitch. To overcome the problem extended and prolonged baking cycles are required. In the highly energy intensive market place and especially with the high cost of energy, this is perceived as a major drawback. Ways to cut down on energy requirements for the manufacture of electrodes are being pursued. For example, U.S. Pat. No. 4,431,503 describes specific improvements in the manufacture of pre-baked carbon containing electrodes for the aluminum industry which allow a reduction on energy requirements. In the aluminum industry, electrodes are of two types: pre-baked electrodes and the continuous self-baking electrodes made with carbon paste. The process for the manufacture of these electrodes consists essentially of preparing a "coke" and a binder mixture called a green mixture. For pre-baked electrodes, the green mixture is shaped and fired. For continuous self-baking electrodes such as Soderberg anodes, the green mixture is fed directly to the operating anode of the electrolytic furnace.
The pre-baked electrodes of U.S. Pat. No. 4,431,503 are made with a binder containing approximately equal weight amounts of a liquid phenolic resole resin and coal tar pitch. As described in Example 4 of U.S. Pat. No. 4,431,503, it is evident that a combination of the phenolic binder and coal tar pitch is essential for adequate performance.
In addition to pre-shaped articles such as the anodes, cathodes, and electrodes, coal tar pitch is also traditionally used as a bonding agent in the process industries mentioned above and in tamping and ramming mix compositions. These compositions are used to form monolithic shapes as well as to conduct on site repair work of refractory linings and the like. In these applications, it is essential that the coal tar pitch is pre-fired to insure carbonizing of the pitch and to insure that the thermoset stage is achieved. Pre-firing must be done during downtime of the work station. The downtime results in a loss of productivity. Additionally, improper heating cycles during the pre-firing steps may lead to an excessive flow of the coal tar pitch. This may lead to even greater health and safety hazards.
Therefore be it in the manufacture of anodes, cathodes, and electrodes, or in the use of binders in making monolithic shapes, workers in the working environment have to contend with the considerable amount of organic fumes emitted by the coal tar pitch. This is highly undesirable for personal hygiene reasons as well as health hazard considerations. The industry recognizes the problem but in the absence of suitable alternative binder systems, can only try to cope with the situation. Industry continues to live with the health hazards of coal tar pitch.
The traditional pre-baked anodes, cathodes, and electrodes are made from a green mixture containing as a binder about 15-30% of coal tar pitch, by weight based on the weight of the mixture of aggregate and binder. Efforts in reducing the amount of coal tar pitch have reportedly resulted in inferior performance of the shaped articles. The prior art indicates that when the then available phenol formaldehyde resins were used as bonding agents in the electrode applications, resin levels of 20%-50% are required for reasonably satisfactory performance. Published literature as described in Encyclopedia of Chemical Technology edited by Kirk and Othmer 2nd Edition, Volume 4, pages 158-243, and Carbon and Graphite Handbook by Martell, Chapters 14, 15, 16 and 17 refer to the use of phenolic resins but report inferior performance properties.
There are other problems that the industry faces because of using coal tar pitch. There are problems due to the fluctuations and variations in the quality of coal tar pitch. Another problem is the presence of undesirable elements such as sulfur and chlorides in pitch which impair the electrical properties of the anodes, cathodes, and electrodes as well as the lives of the electrolytic cells.
Referring now specifically to the aluminum anode industry, thermal shock resistance of the anode is a very critical performance parameter. Another critical parameter is baked density of the anode. Density directly influences such properties as strength, erosion resistance, and electrical resistivity of the anode. A related parameter is the porosity of the anodes. Increased levels of porosity can detrimentally affect the critical performance parameters of the anodes.
The pre-baked anodes for the aluminum industry are made from a green mixture containing as binder about 15%-20% coal tar pitch of a selected grade which is defined by specific solubility level requirements in quinoline. Coal tar pitch levels of as high as 30%-35% are used in the Soderberg process. Extensive fume evolution and loss of volatiles lead to poor densities. These drawbacks have rendered the Soderberg process most obsolete. Even in the pre-baked anode manufacture, porosity is a major problem and needs to be strictly controlled.
In the manufacture of the pre-baked anodes, problems in fracture of anodes due to extensive vapor release, and the problem of extensive energy consumption due to the prolonged and slow baking cycle as well as the resultant loss in productivity are key factors worthy of consideration.